Cracking that takes place in wood during drying is mainly due to the moisture gradient developing inside the wood that generates stresses within the wood because wood contracts non-uniformly when drying. The dried surface layer of wood shrinks more than the moister heartwood, resulting in cracks in the surface. Additionally, there is always a degree of micro-cracking involved when wood dries, which, if uncontrolled, will propagate and develop into macro cracks. The prevention of cracking or the limitation of the size of the cracks to the preferred level is of great importance financially.
Electrical methods have been widely used for the determination of wood moisture content because the effect of moisture on the electrical properties of wood is significant. At low moisture levels, for example, the electrical properties of wood change exponentially rather than linearly as a function of the moisture content. Two commonly used methods for measuring the moisture content of wood is the determination of the electrical resistance of wood and the measurement of the dielectricity of wood. When ohm-meters are used, the electrodes are inserted into the wood, meaning that the material must be partially broken. With measuring methods based on dielectricity, surface contact is usually enough, which makes this method non-destructive. In reality, the reliability of meters relying on surface contact is poor, which is largely due to the fact that the method is extremely sensitive to surface moisture.
The maximum limits for the measuring range of ohm-meters are determined by the saturation point of the wood fibres. At moisture contents exceeding the fibre saturation point, the cell cavities of wood contain free water in addition to the water bound to the grains, and thus the electrical resistance of wood does not, beyond this point, any longer really change as a function of the moisture content. With methods based on dielectricity, it is possible to measure wood moisture levels at moisture contents exceeding the fibre saturation point up to over 100%. In theory, the dielectric constant of wood increases until the cell cavities are completely filled with water. In reality, the accuracy of dielectric hygrometers decreases when the moisture content exceeds the fibre saturation point, which is, among other things, due to the fact that moisture distribution is usually great and a method relying on a single measurement frequency is highly sensitive to variations in the wood surface moisture content.
With commonly used surface hygrometers, the measurement is carried out at a single frequency, which makes the instruments sensitive to surface moisture. While this sensitivity changes as a function of frequency, it is considerable irrespective of the frequency used. If the, specimen being measured exhibits moisture distribution, it is not possible to give an accurate evaluation of the moisture content deeper in the specimen or of the average moisture content in the whole specimen based on the reading provided by the surface hygrometer.
Several scientific studies have shown that an acoustic emission method (hereinafter the AE method) can be used for monitoring the creation and propagation of cracks in various materials, including wood. Acoustic emissions are stress wave impulses generated on release of small amounts of energy, for example in connection with the growth of cracks and the deformation of material. Typically, acoustic signals of the ultrasonic frequency propagate in the material and are reflected and attenuated as well as generate various waveforms.
Normally, emission signals are detected with piezoelectric ultrasonic sensors that convert mechanical vibration into electric signals. The weak pulse received from the sensor is amplified with a pre-amplifier that transmits the amplified signal to the measuring system that typically consists of filters, amplifiers, and analyzers. Typical signal parameters to be analyzed are the number and amplitude of the signals. Also, the frequency content of signals has been used to advantage in the analyses.
The AE method has, in particular, been used in condition monitoring applications for machinery and equipment in the process industry, such as plastics and metals. In wood research, the AE method has been used for determining stresses present in living trees, determining the biological decomposition of wood, detecting termites, analyzing the propagation of cracks and crazes, monitoring wood drying, monitoring the drying of adhesives used in the manufacture of composite wood materials, determining the strength of wood in mechanical strength tests, determining the damage to wood caused by working, as well as for evaluating the condition of wood-working tools.
With the AE method, cracking can be measured quantitatively even before any visible macro cracks appear in the wood. The emissions generated by the cracks are proportional to the stresses appearing inside the wood.
The purpose of the present invention is to provide a method for the determination of the stresses occurring in wood when drying in order to overcome a number of drawbacks associated with the current methods. More specifically, the purpose of the invention is to provide a method that allows easy, efficient and reliable monitoring and/or control of wood drying by means of determining the stresses acting on the material.